This invention relates generally to a voltage clamp circuit and, more particularly, to a circuit for limiting the voltage which appears across the gate and source terminals of a power MOS field effect transistor (MOSFET).
It is well known that automobile manufacturers are tending towards the use of a multiplexed wiring scheme as opposed to the more traditional discrete wiring arrangement. This not only reduces the amount of wiring and connections necessary, but also reduces the labor of troubleshooting and improves long-term reliability (e.g. avoids shorts due to vibrations).
In addition, the automobile industry recognizes the need for diagnostics; i.e., the ability of an onboard computer to sense, for example, when a lamp is burned out or shorted, which lamp is burned out, and immediately inform the driver of the vehicle. Therefore, the multiplexed wiring system must include circuits which are capable of sensing intelligence at each element (e.g. lamp) and distribute this intelligence.
While mechanical switches are used to drive lamps in a discrete wiring system, a multiplexed wiring system requires low current switching such as dash-mounted keyboard or a dusk-on dusk-off sensor, and an electronic power switch such as a TMOS power MOSFET to drive the lamp. Of course, the MOSFET is power dissipation limited; i.e., it can handle a certain amount of power and no more. If the load (lamp) were to become shorted, the entire battery potential would be placed across the terminals of the MOSFET absent certain precautions. Thus, in the case of a load fault, the gate-to-source voltage must be fixed to inherently current limit the MOSFET to prevent it from becoming damaged.